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Need to constantly prime my ROS 125 engine?


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I'm getting really fed up with my ROS 125 engine that I have on my Backbone.

In order to prime and start my engine, I need to:

- Stick my finger into a hole on the side of the carburetor, which presses on an inner membrane that allows the fuel to be drawn into and through the carb into the engine. 

- If I don't press this inner membrane, fuel cannot flow into the engine and priming is impossible. I can squeeze the primer bulb all I want but no fuel will get to the engine. 

- In order to prime it, I need to put my finger into the hole to press on the inner membrane and then give a small squeeze of the primer bulb.

- I HAVE to do this with the motor off my back because the inner membrane hole which allows fuel to flow through the carb is impossible to reach with the unit on my back. 

- My entire motor unit is 38kg with fuel.

So here are my problems:

- When I start the engine for the first time, I prime it but then I'm heavily incentivized to start it on the ground instead of on my back. The reason is that once I put it on my back (and hook up FIVE straps), if the engine doesn't start, I have to undo all the straps, take the unit off my back, prime it again by pressing on the membrane, put it back on my back, buckle five straps, and retry. It is a massive, massive pain in the ass.

- The engine becomes "unprimed" after just 5-10 minutes of sitting around. Say that I prime the engine and get it started and then kill it. I get my wing and lines sorted in the next five minutes but then the wind direction suddenly changes. So I take all my stuff and walk a couple minutes over to another corner of the landing field. I then try to start the engine and it won't start anymore, despite having started 7 minutes ago. I then have to unbuckle EVERYTHING, take the thing off my back, put my finger into the unreachable f*&(ing inner membrane, squeeze the primer bulb, get it running on the ground, kill it, put it on my back, buckle all five straps, only to have the wind change direction on me again.

So here are my questions:

- What motor do you have and can you describe the step-by-step process of starting it as I have with mine?

- I'm probably going to ditch this ROS125 and go with whatever engine is light and starts easily and reliably on your back.

Yesterday was absolutely killer for me. I had to take the unit off my back twice to re-prime because it refused to start when I was ready to launch having walked around the field as the wind changed. As a result, I was so tired and flustered on the last attempt that I forgot to buckle one of my FIVE straps. By the time I took off, my mirror which was attached to the end of the unbuckled strap flew into the prop and got eaten up. Luckily the prop didn't break (it just got rashed up a little) but now I'm out a mirror not to mention I had an incredibly frustrating and tiring time launching.

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Hi, really sorry to hear of your problems.

I have a Bulldog with a Polini 100 (110cc). I literally do up 4 straps, prime it on my back with 4 pushes of the bulb which is easily reachable, grad the starter cord which is attached to the shoulder strap by a magnet and then no more than a couple of pulls of the cord later she will start from completely cold.


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It's not the primer bulb, but rather the membrane on the carb I *have* to push on with my finger to let fuel into the carb and to the engine. If I don't push it in, the primer bulb is rock hard because the carb is essentially blocked. Pushing on the membrane unblocks the carb and allows the bulb to squeeze and fuel to go through.

There is no way for me to prime the thing while on my back because I simply can't reach my hand back far enough to press on that membrane. 

So to be clear, *no one* has this kind of setup? You can prime the engine and everything while it's still on your back? Is my ROS125 the only engine that has this?

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Yes it is normal, you have to depress the diaphragm button to lift the needle valve to allow fuel into the carb to prime.

just get to know how much to prime on ground then put motor on and start on your back. 

I used to start for few seconds on ground the stop and stick it on my back and restart.  But I've changed now and only start on my back,

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On 8/18/2016 at 21:38, GR002 said:

Replace the primer bulb as well as it has two one way valves in it and should not allow fuel to run back to tank. There are good quality primer bulbs on ebay.

Like said fuel should not be draining back especially that quick, so you have a fuel supply problem. 

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